9nppffTill that time there is overflow of oil to the suction side. By moving upwards, depending upon the load of engine, there is relative position of plunger helix with respect to the suction. This will decide as to when the discharge side of the plunger will be in connection with the suction side, hence ending the delivery. Also this will decide quantity of fuel injected depending upon the load. This system has constant beginning but the end and quantity of injection is controlled by the helix position.
Q.31 Same as Q.30
Q.32 An individual pump is designed for variable beginning and constant end of injection. For diesel engine operating at constant speed, the start of injection will
Ans. advance as the load increases.
As the load increases, engine requires more fuel to inject and burn. As the speed is constant, more fuel is to be injected in same time frame available, it is necessary to begin the injection earlier, i.e. to advance the beginning of injection, so that the fuel can be completely burnt, effectively and efficiently, within the time frame available thus giving better thermal efficiency of the engine.
Q.33 Crankshaft whose center of gravity coincides with the centerline is said to be
Ans. (b) statically balanced.
B012
Ans. (b) increase the distance of heat travel from the crown to top ring groove.
As the first ring or the top ring is subjected to maximum amount of temperature and pressure, it is difficult to maintain the film of lubrication between the top ring and liner. Also there is every possibility that due to heat and high pressure, the top ring will get broken or seized in the piston groove. Due to lack of lubricating oil film, there is maximum weardown for the top ring and top end of the liner. In order to avoid this, it is necessary that the piston ring should be maintained as far as away for the firing temperature so that a good lubricating oil film can be maintained between the liner and piston rings. In order to achieve this, a seat damp is utilized in the engine.
Q.40 Diesel engines are classifieds as internal reciprocating engines because they
Ans. (c) burn the fuel in chamber and their energy moves the piston back and forth.
As the fuel is burning in confined space which is sealed where fuel burning takes place because of high temperature of compressed air. All the burning process is carried out inside the cylinder liner between the top of the piston and bottom of the cylinder head. The heat released is converted into mechanical energy by the movement of the piston which is transmitted to the crankshaft. As the combustion takes place inside the cylinder, it is called internal combustion engine.
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Q.53 Turbulence in diesel engine cylinder is of major importance in providing
Ans. (c) complete fuel air mixing.
Turbulence is created due to the circular movement of air in the cylinder. This turbulence is created by the geometry of entry ports, scavenge air as well as the geometry of the cylinder head and the piston crown. Due to turbulence there is thorough mixing and homogenize mixture is produced between fuel and air. This mixture will burn efficiently and hence the full quantity of air is available for the combustion of fuel. This gives higher thermal efficiency of the engine and reduce the unburnt fuel going into the exhaust thus reducing the specific fuel oil consumption and less smoke is formed in the exhaust. At the same time, this will give the uniform distribution of heat over the combustion space.
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Q.54 Load is always placed on lower half of main bearing in
Ans. (a) two-stroke cycle engine.
In two-stroke engine, as there are two strokes for the complete cycle, the piston will always be loaded downwards whether it is expansion or compression stroke. Hence for first half revolution, the piston is loaded downwards due to expansion gases which is transmitting load force to the crankshaft. When the piston is moving upwards it is compressing the air to perform compression stroke. Due to this unidirectional downward loading of the piston which is transmitted to the crankshaft, the main bearing of the two-stroke cycle engine is always loaded at bottom half. Also the pressure acting is varying and hence difficulties are encountered in lubrication of bearings in case of two-stroke engines rather than four-stroke engines.
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Q.55 Cavitation and corrosion in cooling water system of diesel engine usually occurs at the pump impeller and the waterside of the
Ans. (c) cylinder liner.
When the water enters into the cylinder liner, it gets heated up. At the same time when water passes through the cylinder liner, the velocity of water increases due to restricted passages. As the velocity increases, as per Bernoulli’s theorem, pressure drop takes place. As the pressure drop takes place, water tends to boil off at lower temperature due to which steam bubbles are formed. These bubbles get bursted creating cavitation. Hence if the liner is opened for inspection any time, especially at the places of restricted entries and passages, cavitation is easily visible.
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Q.56 The main operating characteristics that distinguishes it from other internal combustion engines is
Ans. (c) method of igniting the fuel.
In petrol engines, the ignition is carried out with the help of igniter or spark plug. Diesel engine fully depends on compression ignition for the purpose of igniting the fuel. In diesel engine the air is compressed. As the air is compressed its temperature increases to a temperature which is more than the self-ignition temperature of the fuel. When fuel is injected at this temperature, it vapourizes and mixes with air to form combustible mixture. As the temperature is above the self-ignition temperature this mixture ignites and produces a high pressure wave. This pressure wave and the heat is transmitted to the crankshaft by the downcoming piston.
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Q.57 Which term describes piston pin having bearing surface in both piston bosses and connecting rod eye?
Ans. (b) Full floating.
If the gudgeon pin is having a press-fit in the bosses of the piston and is free to move over the top end bearing of the connecting rod, there is a possibility that misalignment cannot be taken care off. Hence the pin is made to float or have bearing surface in the piston boss and the connecting rod eye so that it can take care of misalignment. At the same time the pin can have a rotating effect. When the pin has rotating effect, it will not have continuous loading on one surface and the weardown of the bearing will be uniform thus preventing the ovality of the bearing due to wear in only one place. In order to prevent this pin from coming out when the piston is moving up and down, at the end of bosses, circlips are provided which prevents the gudgeon pin from coming out and touching the liner.
B0021
Q.58 Compared to other fuel injection systems, unit injection system operates with virtually no
Ans. (a) injection lag.
In unit injection system, the fuel pump and the fuel valve consists of a single block. Hence there is no fuel piping the valve and the pump. As there is no piping, the time required for injection from the time the fuel pump plunger starts moving up is reduced resulting in reduction of injection lag. In case of other types of fuel pumps, where the fuel pump is connected to the injector by long high pressure pipes, some amount of pressure drop takes place inside the fuel pump piping. Also due to the pressure wave created, the fuel piping gets slightly expanded which can cause fuel injection delay. Hence in order to avoid fuel injection delay, the best possible way is to fit the fuel pump very near to the injector. In unit fuel injection system, this delay is nearly reduced to zero.
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Q.59 Piston cooling fins are located
Ans. (b) underneath the piston crowns.
Piston cooling fins act like cooler. When the fins are provided, better heat transfer takes place from the hot surface towards the pointed tips of the fins due to which the hot crown remains comparatively cool and better heat transfer takes place across the cross-section crown. Hence the piston crown induced thermal stresses are reduced to minimum. This has resulted in less cracking of the piston crowns.
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Q.60 Properly honed diesel engine cylinder liner will
Ans. (b) shorten the ring breaking in period.
When the liner and the piston rings are new, microscopic * and roughness is always present in the liner wall and piston ring surface. During the breaking in period, these * are grounded or broken up so that they will be smoothened. Running in can be done afterwards to achieve complete bedding between the liner and piston rings. Hence breaking in is importance process. If it is not done properly, there is a possibility of scuffing and blowpast. The process of breaking in should be carried out as fast as possible so that the engine load can be increased. In order to get fastest breaking in, controlled fast weardown should be achieved. For this purpose, honing should be carried. During honing process, the surface of the liner is made artificially rough. Thus lubricating oil is retained at the surface of the liner. Also due to roughness, both piston rings and liner will get worn out faster so that breaking in can be carried out fastly, without causing blowpast or scuffing. Hence honing is important from the point of view of breaking in.
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Q.61 Besides the use of piston cooling, fins provides extra strength for the
Ans. (b) piston crowns.
As the fins are provided on the underside of the piston crown, the top portion of the piston crown which is subjected to higher temperature and pressure is supported by the fins from underside. As the support is provided by the fins, they give extra strength to the piston crown. Hence it is possible to use the piston crown of thinner cross-section without causing it to buckle under the influence of high temperature and pressure.
B0026
Q.62 Minimum fuel oil delivery pressure required for efficient fuel injection depends primarily on
Ans. (b) maximum pressure in the engine cylinder during injection.
When the piston is approaching TDC on compression stroke, the fuel oil injected into the cylinder. The fuel oil injected has to overcome the resistance or backpressure offered by the high pressure compressed air. Hence the pressure of the fuel oil injected has to be more than the pressure inside the cylinder head; otherwise it will not be possible for the fuel to penetrate the mass of dense air available. This will result in improper combustion of the fuel. Hence it is necessary to maintain minimum injection pressure higher than the compression pressure existing at the time of injection.
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Q.63 A thrust bearing is used in diesel propulsion engine to
Ans. (a) control axial movement of the crankshaft.
When the ship moves forward or aft, the propeller produces a thrust which is applied to ship’s hull. At the same time this thrust is exerted on the propeller blades. As the propeller is connected to the propeller shaft which is connected to the engine shaft, due to the axial thrust created, there is a tendency for the crankshaft to move inside the engine i.e. the crankshaft will have axial movement. If this axial movement is not prevented or taken care off, there will be misalignment between the piston connecting rod and bottom end bearing. Also the crankshaft webs may touch the A-frame and the main bearings which may damage to the engine structure. Also the rest of the crankshaft will be subjected to axial thrust and hence it may bend or break. Hence it is necessary to take care of this axial thrust before it enters the engine and affects the moving parts of the engine. For this purpose, thrust block is provided at the end of the last unit so that all other units and crankshaft is protected from the axial thrust and the crankshaft alignment will not be affected.
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Q.64 The two strokes of the two-stroke cycle diesel engines are
Ans. (d) compression and power.
As any diesel cycle, the two-stroke cycle will also consist of all four major processes i.e. suction, compression, power and exhaustion. In case of two-stroke engines, the process of compression and power are distinct but air intake and exhaustion are combined together and they form a part of the distinct compression and power stroke. Hence there is no separate distinct stroke for intake and exhaustion as exhausting is carried out by intake of air.
B0030
Q.65 Which of the following statements pertains to propulsion engines with bore exceeding 200mm?
Ans. (c) The units will be fitted with means to display the exhaust temperature of each cylinder.
When the piston diameter exceeds 200mm, the engines are considered as highly rated engines. In such engines, the exhaust temperature of individual units will be on higher side and sufficient amount of power is developed by the units. In order to have proper running of the engine, it is necessary to have a balanced engine. The balanced engine can be roughly described by comparing the individual unit temperature. If the exhaust temperatures of the units are within prescribed range, one can consider that the engine is properly balanced. It should also be noted that exhaust temperature is not the only indication of power balance. For this purpose, the indicator card is taken and the power developed by the individual unit is calculated the best method. It can be a rough estimation of power balance.
B0031
Q.66 When engine bore exceed 230mm, a berthing disc or flame arrestor is fitted
Ans. (a) to the engine air supply inlet to the starting air supply manifold for non-reversing engines.
When the bore of the engine exceeds 230mm it is considered as highly rated engine as far as power is concerned. These engines are mostly started with compressed air. If air inlet valve or starting air valve on the cylinder leaks, high pressure exhaust gas will enter the air manifold. As the pressure and the temperature is high, it will cause a air starting line explosion. This will result in heavy damage to the engine and the personnel working. Also the engine will be completely immobilized.
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